1. Field of the Invention
The invention relates to a method and a device for operating a photovoltaic system having an inverter at whose input terminals the DC output voltage of the photovoltaic system is applied and whose output can be connected to a supply network, and having a battery bank connected in parallel to the input terminals, whereby the inverter is provided with a regulating element for adjusting the maximum power point (MPP) of the photovoltaic system.
2. Description of the Background Art
During the construction of solar power plants, it is provided sometimes in the supply agreements with the associated electric utility to supply a contractually guaranteed minimum power within an established time interval to the supply network. Thus, for example, it can be required for a 2.4 MW (Mega Watt) solar power plant that a power of at least 60%, therefore about 1.5 MW, is to be supplied between 11:00 a.m. and 5:00p.m. Depending on weather conditions, the minimum power often cannot be generated by the solar plant alone, and replacement power plants on a DC (Direct Current) basis, such as, for example, a battery or a fuel cell, must be connected in addition. In a prolonged predicted energy reduction, an additional power plant must then be called upon on the AC (Alternating Current) side. This occurs either by the additional purchasing of the amount of energy from another energy supplier or by connection of an additional power plant on the AC side, e.g., in the form of a diesel generator or a gas power plant. The possible energy suppliers have different startup times until their power can be fed in. This ranges from virtually immediately during the discharge of a battery up to about 5 minutes for the starting of a supplementary power plant.
FIG. 1 shows a plant in which a photovoltaic generator (PV generator) 1 is connected in a typical manner to the input terminals 3, 3′ of an inverter 5 to whose output a supply network 7 is connected. A second inverter 5′, whose DC voltage side leads to a battery or battery bank 9, is tied into supply network 7. If the power produced by the PV generator is not sufficient, the battery is discharged via the second inverter 5′ into the network and in this way supports the low-performing PV system 1. The discharged battery 9 is then charged by means of the line voltage. A disadvantage in this arrangement, on the one hand, is that two inverters 5, 5′ are required, which increases investment costs. On the other hand, electrical losses occur in the second inverter 5′ during its charging from the supply network 7 and these are also associated with costs. As a result, two inverters 5 and 5′ are used to recharge the battery with direct current generated in the PV system.